1. Field of the Invention
This invention relates to the packaging of electric circuit devices, such as microminiature integrated circuit chips. In particular, it relates to the dissipation of heat generated by the chips.
2. Description of the Prior Art
The dissipation of heat from a semiconductor chip is a major problem in the industry. As more and more transistors and other devices are fabricated from within the semiconductor chip, the amount of heat which is generated during the electrical operation of the chip increases proportionally.
Semiconductor designers have long been aware of the need for removing the heat and have devised numerous ways to do so. Generally, the techniques can be segregated into two basic means internal to the module: air cooling and liquid cooling. The latter technique usually involves placing the chip packages in a bath of low-boiling-point liquid such as fluorocarbon fluid, for example. This process is very efficient but raises problems with respect to the contamination of the devices by the liquid, leakage of the liquid from the container which could cause catastrophic failure, and increased manufacturing costs.
Air cooling, which generally involves contacting one or more surfaces of the semiconductor chip with a good heat conducting element such as copper, is cheaper, cleaner and unlikely to create problems of the aforementioned catastrophic failures. However, air cooling by simple, direct contact of the heat conductive element to the chip may not conduct sufficient heat away from the chip, due to the imperfect, noncompliant nature of the contact; in addition it imposes stresses within the chip and its interconnecting joints due to the direct transmission of forces caused by thermal expansion and contraction, mechanical disturbances, etc.
Air-cooled assemblies usually involve metallurgically bonding the semiconductor chip to the heat conductive cap, which is also used for sealing the chip. Packages of this type are illustrated, for example, in the articles entitled, "Chip Heat Sink Package Assembly," by Johnson et al, IBM Technical Disclosure Bulletin, March 1970, p. 1665, and "Conduction-Cooled Heat Plate for Modular Circuit Package", Dombrowskas et al, IBM Technical Disclosure Bulletin, July 1970, p. 442. Although effective in removing heat from the chip, such structures involve metallurgical bonds both between the heat sink and the semiconductor chip as well as the heat sink and the conductive sealing cap. Such structures may subject the chip and the chip joints to undue stresses during thermal expansion or contraction.
In addition, reworking capability is particularly important for packages in which a plurality of chips are mounted on a single substrate and enclosed by a single cover. It is often necessary to replace one defective chip out of many or to repair the wiring on the substrate. Bonded connections, however, cannot be disassembled to allow rework or repair.
Other packaging designs have recognized the need to provide both high thermal conductivity as well as the ability to absorb mechanical stress. See, for example, the article entitled, "Conduction-Cooled Chip Module," Dombrowskas et al, IBM Technical Disclosure Bulletin, February 1972, p. 2689. The article suggests the use of pads of conductive dispersion material which never cure or completely harden to fill the space between the chips and the heat sink cover. Such material, however, results in too high a thermal resistance to be practical and may be corrosive.
The above-referenced, related application by Koopman and Totta solves this problem by providing a readily deformable metal or alloy such as indium and mechanically deforming the mass of metal against the back side of the chip after metallurgically bonding the metal to the inside of the cover. This results in a structure which not only provides an excellent heat transfer path from the chip to the cover but also results in little or no stress on the chip or its interconnecting joints during circuit operation. As discussed in the above-identified application, this technique has been very effective in solving this major problem. However, their method requires that the force exerted on the metal pad be carefully controlled so as not to exceed the limits which can be endured by the chip joints or the substrate.